we demonstrated a diode-pumped continuous wave and passively Q-switched Nd:LuxY1-xVO4 laser at 1.34 μm with V3+:YAG as the saturable absorber. The crystal Nd:LuxY1-xVO4 with equal ionic ratio (x = 0.5) shows better laser performance. The maximum continuous wave output power of 1.45 W was obtained with the optical efficiency of 20.1% and the slope efficiency of 24.5%. For the pulsed operation, the minimum pulse width achieved was 42 ns with the pulse repetition frequency of 142 kHz, and the single pulse energy and the peak power were estimated to be 6.62 μJ and 157.6 W, respectively.
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Compared with single laser crystals, mixed crystal materials, usually with moderate stimulated emission cross-section and large fluorescence bandwidth, are very attractive for Q-switching and mode-locking of solid-state lasers [1–8]. The moderate stimulated emission cross-section is helpful to suppress parasitic oscillation, as well as the large fluorescence bandwidth is benefit to generate short pulse duration. Liu  has realized 2.78 W average power with a slope efficiency of 45.5% from PQS Nd:Gd0.64Y0.36VO4 laser. Our group demonstrated 3.1 W average power output and the shortest pulse width of 7.8 ns from a PQS Nd:Lux(x = 0.15)Y1-xVO4 crystal laser, which showed the widest fluorescence bandwidth in the group of mixed crystal Nd:(Lu,Y, Gd)VO4 . However, most of attention was paid to 1.06-μm laser operation, and only few research of mixed crystal was on the 1.34 μm laser source, which has wide applications in the fields of medicine, communications and light-sensing [10,11].
Passively Q-switched lasers have many advantages of simplicity, high efficiency, and low cost without the need for high-voltage and RF drivers [12,13]. As for 1.34 μm pulsed operation, V3+:YAG has some remarkable advantages owing to their excellent physical and optical performance at 1.34 μm, such as high damage threshold, high ground state absorption cross-section of 7.2 × 10−18 cm2, and low residual absorption at 1.34-μm [14,15]. Liu et al. reported 1.02 W PQS c-cut Nd:LuVO4 laser operation with V3+:YAG as a saturable absorber . For the Nd-doped mixed crystal, Huang et al. have realized a passively Q-switched Nd:Gd0.5Y0.5VO4 laser at 1.34 μm and the maximum average output power of 0.96 W was achieved with the minimum pulse width of 47.8 ns . Li et. al reported a diode pumped passively Q-switched Nd:Lu0.15Y0.85VO4 laser at 1.34 μm with V3+:YAG as a saturable absorber for the first time and the pulse width of 85.6 ns was attained .
In this paper, the characteristics of mixed crystal Nd:LuxY1-xVO4 with equal ionic ratio (x = 0.5) is investigated. We successfully demonstrated the continuous wave (cw) and PQS laser of Nd:Lu0.5Y0.5VO4 crystal with V3+:YAG as a saturable absorber for the first time to the best of our knowledge. For the Nd:Lu0.5Y0.5VO4 mixed crystal, the maximum cw output power was 1.45 W under the pump power of 7.23 W, corresponding to the optical efficiency of 20.1% and the slope efficiency of 24.5%. For the 1.34 μm pulsed laser operation, the maximum average output power of 0.94 W was obtained, with the shortest pulse width of 42-ns and the repetition frequency of 142 kHz. The single pulse energy and peak power was estimated to be 6.62 μJ and 157.6 W, respectively. Meanwhile, the characteristics of the mixed crystal with a low ionic ratio (x = 0.15) was also compared. We obtained 1.24 W cw and 0.88 W average power passively Q-switched output from Nd:Lu0.15Y0.85VO4 crystal lasers. The minimum pulse width obtained was 81.8 ns with the pulse repetition frequency of 150-kHz. The results indicate that the Nd:LuxY1-xVO4 crystal with 0.5/0.5 ionic ratio shows better laser performance for passively Q-switching at 1.34 μm, in comparison with the crystal with 0.15/0.85 ionic ratio.
2. Experimental Setup
A schematic diagram of the experimental apparatus was shown in Fig. 1 , which was based on a compact plano-concave resonator. The crystals Nd:Lu0.5Y0.5VO4 and Nd:Lu0.15Y0.85VO4 used in our experiment were grown by Czochralski method, with the same Nd-doping concentration of 0.38%. The crystals were cut along a-axis with the same dimensions of 3 × 3 × 10 mm3. The end faces of the samples were polished and antireflection (AR) coated at 808 nm (T>99.5%). The AR coating was not optimized for the wavelength at 1.34 μm and the transmission was estimated around 97%. The crystals were wrapped in indium foil and mounted by a copper micro channel heat-sink, respectively, to release the heat deposition efficiently. The pump source employed in the experiment was a fiber-coupled LD with the central wavelength around 808 nm at room temperature. The fiber core was 400 μm in diameter with a numerical aperture of 0.22. Through the focusing optics, the pump beam was focused into the laser crystals with the radius of about 200 μm. The concave mirror M1 with the curvature radius of 200 mm was AR coated at 808 nm and 1.06 μm while high reflection (HR) coated at 1.34 μm. The output coupler M2 had different transmissions with 8% and 15% at 1.34 μm. A V3+:YAG absorber was used, with the dimensions of 5 × 5 × 0.5 mm3 and the initial transmission of 94% at 1.34 μm. A beam splitter (M3) was positioned behind the output coupler, making it possible to measure the output power and the pulse simultaneously. The lengths of the cavities in the experiment were 35-mm.
3. Experimental results and discussions
In our experiment, the cw laser operation was studied firstly. And the PQS laser operation was carried out after the cw experiment by inserting the V3+:YAG absorber into the cavity. The cw Nd:LuxY1-xVO4 laser characteristics at 1.34 μm and fluorescence spectrum was shown in Fig. 2 . The maximum laser power of 1.45 W was achieved under the pump power of 7.23 W and the output coupler with 15% transmission for the Nd:Lu0.5Y0.5VO4 crystal, resulting in an optical conversion efficiency of 20.1% and a slope efficiency of 24.5% (the threshold pump power was 1.3 W). The peak of laser spectra was at 1.342 μm with the FWHM bandwidth of 2-nm while the bandwidth of fluorescence spectra at 1.343 μm was 3 nm. And the beam quality parameter M2 was measured to be 1.8 at the maximum output power. Higher output power will be obtained if the AR coatings at the crystal end faces were optimized at 1.34 μm. For the Nd:Lu0.15Y0.85VO4 mixed crystal, the highest output power was 1.21 W under the same condition, corresponding to the optical efficiency of 17.2% and the slope efficiency of 20.9%. When the output mirror was replaced by another with 8% transmission, the maximum cw laser power were 1.21-W and 1.18-W for the Nd:Lu0.5Y0.5VO4 and Nd:Lu0.15Y0.85VO4 crystal, respectively. Because of its slight larger emission section at 1.34 μm, the threshold pump power of Nd:Lu0.15Y0.85VO4 is lower than that of Nd:Lu0.5Y0.5VO4. However, the Nd:Lu0.5Y0.5VO4 crystal shows more excellent cw laser performance in the higher pump power.
To achieve efficient passively Q-switched laser performance, it is necessary to make sure that absorption saturation in the absorber occurs earlier than that in the gain (usually be called the second threshold condition) . Considering the large ground state absorption cross-section (σgs = 7.2 × 10−18 cm2) of V3+:YAG, smaller stimulated emission cross-section (σ = ~1.1 × 10−19 cm2) of the Nd:LuxY1-xVO4 crystal, and the ratio between the effective area in the crystal and in the absorber (A/As≈1) in our experiment, the second threshold condition can be satisfied easily.
Figure 3 described the average output power of passively Q-switched Nd:LuxY1-xVO4/V3+:YAG at 1.34 μm. The best performance was obtained with t = 15% output coupler. For the Nd:Lu0.5Y0.5VO4 crystal, the maximum average output power was 0.94-W at the pump power of 7.23 W, corresponding to the optical efficiency of 13% and the slope efficiency of 18.2% (the threshold pump power was 2.07 W). The corresponding Q-switching efficiency (ratio of the Q-switched output power to the cw output power at the maximum pump power) was 64.8%, which is much higher than that of Nd:Gd0.5Y0.5VO4 in Ref . The highest average output power was 0.88 W for the Nd:Lu0.15Y0.85VO4 crystal, which is lower than that of Nd:Lu0.5Y0.5VO4 crystal. With the output coupler of t = 8%, the maximum average output for the Nd:Lu0.5Y0.5VO4 and Nd:Lu0.15Y0.85VO4 crystal were 0.65 W and 0.55 W at the pump power of 7.23 W, respectively.
The repetition rates versus pump powers for the PQS lasers were displayed in Fig. 4 . As can be seen, the pulsed repetition rate increased monotonically with the augment of the pump power. For Nd:Lu0.15Y0.85VO4 crystal, the value increased from 49 kHz to 150 kHz, with the output mirror of t = 15%. On the other hand, when the output mirror of t = 8% was exploited, the pulse became unstable when the pump power exceeded 6.54 W. This may be because thermal induced fluctuation in the Nd:Lu0.15Y0.85VO4 crystal under high pump power. This phenomena didn’t appear for Nd:Lu0.5Y0.5VO4 crystal. When the t = 8% and 15% output couplers were exploited, the pulse repetition rates of the Nd:Lu0.5Y0.5VO4 crystal increased from 62 to 280 kHz and 40 to 142 kHz, respectively, with the pump power increasing from 2.07 W to 7.23 W.
Figure 5 depicted the dependence of pulse widths on the pump power. It can be also found that, the pulse width decreased sharply at the lower pump power, while it became gradually with increasing the pump power. Meanwhile, the shortest pulse width of 42 ns was obtained with the Nd:Lu0.5Y0.5VO4 crystal by the t = 15% output coupler, corresponding to the single pulse energy of 6.62 μJ and peak power of 157.6 W. While under the same condition, the minimum pulse width of 81.8 ns for the Nd:Lu0.15Y0.85VO4 crystal was achieved, with the single pulse energy and peak power estimated were 3.52 μJ and 71.7 W, respectively. The results indicated that the Nd:Lu0.5Y0.5VO4 crystal has more significant characteristics in passively Q-switching, which coincided with the result of the Nd:GdxLu1-xVO4 crystal in Ref .
In conclusion, we have successfully demonstrated the cw and PQS laser operation of the Nd:Lu0.5Y0.5VO4 crystal at 1.34 μm for the first time. The influence of ionic ratio between Lu ion and Y ion to laser operation was also investigated. The maximum output power of 1.45 W was achieved at the pump power of 7.23 W with Nd:Lu0.5Y0.5VO4 crystal with the optical conversion efficiency of 20.1% and the slope efficiency of 24.5%, respectively. As for pulsed operation, the minimum pulse width obtained was 42 ns, with the pulse repetition frequency of 142 kHz, and the single pulse energy and peak power was estimated to be 6.62 μJ and 157.6 W, respectively. Compared with the Nd:Lu0.15Y0.85VO4 crystal, the Nd:Lu0.5Y0.5VO4 crystal showed more excellent PQS laser performance.
This work is supported by the National Natural Science Foundation of China (60478018, 20801013), the Science and Technology Development Foundation of Fuzhou University (2009-XQ-13) and the Technology project of the Education Department of Fujian Province, China (JB07003).
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